1. Field of the Invention
The present invention relates generally to a mobile communication system using an orthogonal frequency division multiplexing (OFDM) scheme, and in particular, to a method for controlling operational states of a medium access control (MAC) layer.
2. Description of the Related Art
Since the development of a cellular mobile telecommunication system in the United States in the late 1970's, South Korea has begun providing a voice communication service with an AMPS (Advanced Mobile Phone Service) mobile communication system which can be regarded as a 1st generation (1G) analog mobile communication system. Thereafter, a code division multiple access (CDMA) mobile communication system, a 2nd generation (2G) mobile communication system, was commercialized in the middle of the 1990's to provide a voice and low-speed data service.
In addition, beginning in the late 1990's, IMT-2000 (International Mobile Telecommunication-2000), a 3rd generation (3G) mobile communication system, aimed at providing an improved radio multimedia service, a worldwide roaming service and a high-speed data service, was developed and recently commercialized in part. Particularly, the 3G mobile communication system has been developed to transmit data at higher speed due to an increasing amount of data served in the existing mobile communication system.
Currently, the 3G mobile communication system is evolving into a 4th generation (4G) mobile communication system. The 4G mobile communication system is being standardized with the intentions of providing efficient interworking and a unified service between a wired communication network and a wireless communication network, in addition to the simple radio communication service provided in the exiting mobile communication system. Therefore, it is necessary to develop technology capable of transmitting massive data approximating the capacity of a wired communication network, in a wireless communication network.
With the development of the mobile communication technology, the existing voice-centered service is evolving into a data-centered service, and thus, the mobile communication system is evolving from a circuit switching-based network into a packet switching-based network. The packet switching system assigns a channel only when there is data to transmit, thus causing frequent channel access and release operations. Furthermore, in the packet switching system, its entire system efficiency depends upon an operation method of a medium access control (MAC) layer that manages the channel access and release operations. An operation of the MAC layer will now be described below.
An operation of the MAC layer is determined according to a connection state between a mobile station (MS) and a mobile communication system, and each mobile communication system is unique in operation of its MAC layer. First, an operation of a MAC layer in the 2G mobile communication system will be described with reference to FIG. 1.
FIG. 1 schematically illustrates operational states supported by a MAC layer in a general 2G mobile communication system. Referring to FIG. 1, in the 2G mobile communication system, a MAC layer supports two operational states, i.e., an active state 111 and a dormant state 113. Herein, the 2G mobile communication system refers to, for example, a TIA/EIA-95-B system. The active state 111 represents a state in which there is traffic such as voice data to be transmitted to the mobile station, and downlink and uplink dedicated control channels (DCCH) and dedicated traffic channels (DTCH) are assigned to the mobile station. The dormant state 113 represents a state in which there is no downlink and uplink dedicated control channel and there is no base station (BS) and mobile switching center (MSC) resource. In this state, a point-to-point (PPP) state is held and there is a small amount of data burst.
In the 2G mobile communication system, even though there is no transmission and reception data in the active state, the MAC layer continuously assigns dedicated channels, i.e., a dedicated control channel and a dedicated traffic channel; so the 2G mobile communication system is not suitable for a data service having a burst characteristic. Because radio resources for dedicated channels are assigned to mobile stations even though there is no actual transmission and reception data, the number of mobile stations in the active state, which can be accommodated within a cell, is limited.
FIG. 2 schematically illustrates operational states supported by a MAC layer in a conventional 3G mobile communication system. Referring to FIG. 2, in the 3G mobile communication system, a MAC layer supports an active state 211, a control hold state 213, a suspended state 215, and a dormant state 217. Herein, the 3G mobile communication system refers to, for example, a CDMA2000 system.
The active state 211, like the active state 111 described in conjunction with FIG. 1, represents a state in which there is traffic to a mobile station and downlink and uplink dedicated control channels and dedicated traffic channels are assigned to the mobile station. The control hold state 213 represents a state in which power control (PC) is continuously performed, downlink and uplink dedicated control channels are assigned, and traffic channels can be rapidly reassigned. The suspended state 215 represents a state in which downlink and uplink dedicated control channels to the mobile station are not assigned, radio link protocol (RLP) and PPP states are held, a virtual active set exists, and a slotted submode is supported. The dormant state 217, like the dormant state 113 described in conjunction with FIG. 1, represents a state in which there is no downlink and uplink dedicated control channel, and also, there is no BS and MSC resource. In this state, a PPP state is held and a small amount of data burst exists.
In the 3G mobile communication system, the MAC layer supports the 4 operational states considering not only a voice service but also a data service to assign radio resources only when there is transmission/reception data, thereby improving the entire system performance. However, like the MAC layer of the 2G mobile communication system, the MAC layer of the 3G mobile communication system also must perform a contention-based random access procedure in order to transition from the control hold state 213, the suspended state 215, and the dormant state 217 to the active state 211. The contention-based random access procedure reduces a state transition speed from the other states to the active state 211, causing a decrease in the entire system performance. In addition, in the light of a structural characteristic of logical channels, the number of mobile stations having the control hold state 213 and the suspended state 215 is limited, so the 3G mobile communication system is not suitable for an ‘always on’ requirement, which is one of the major service quality satisfying requirements of a mobile communication system. The term ‘always on’ refers to a state in which contention-free-based random access rather than the contention-based radon access is available with downlink and uplink dedicated channels even in other states excluding an active state.
FIG. 3 schematically illustrates operational states supported by a MAC layer in a 4G mobile communication system, which is currently under discussion. A mobile communication system using an OFDM scheme (i.e., an OFDM mobile communication system) has been actively studied as a 4G mobile communication system. The OFDM scheme transmits data using multiple carriers, and is a kind of a multi-carrier modulation (MCM) scheme for parallel-converting a serial input symbol stream and modulating the parallel-converted symbols with a plurality of orthogonal subcarriers (or subchannels) before transmission. The OFDM scheme is similar to the conventional frequency division multiplexing (FDM) scheme, but characterized by maintaining orthogonality between the subcarriers thereby securing optimal transmission efficiency during high-speed data transmission. In addition, the OFDM scheme has high frequency efficiency and is robust against multipath fading, contributing to optimal transmission efficiency during high-speed data transmission.
In the proposed 4G mobile communication system, a MAC layer supports 5 optional states of an on-state 311, a hold state 313, a sleep state 315, an access state 317, and a null state 319. The on-state 311 represents a state in which data traffic is transmitted and received, there is a full fledged uplink control channel having all control information, and rich QoS (Quality of Service) functionality is supported. The hold state 313 represents a state in which timing is controlled, coarse power control is performed, rapid transition to the on-state 311 is possible on a contention-free basis, there is a thin uplink control channel having only basic control information, users can receive data traffic, and a power save mode is supported. The sleep state 315 represents a state in which no power and timing control is performed, an ultra power save mode is supported, and a large number of mobile stations are supported. The access state 317 is a random access state for channel acquisition, and the null state 319 is identical to the dormant state 217 illustrated in FIG. 2.
The MAC layer of the 4G mobile communication system defines logical channels that utilize characteristics of the OFDM scheme, enables contention-free-based random access in a particular state, and proposes operational states for increasing the number of available mobile stations as compared with the 3G mobile communication system. However, as described in conjunction with FIG. 3, the MAC layer of the 4G mobile communication system also must demand a contention-based random access procedure in order to transition from the other states to the on-state 311. In addition, the number of mobile stations which are available in a state where the contention-based random access procedure to the on-state 311 is not necessary, i.e., in the hold state 313, is limited.
Operational states of the MAC layers in the 2G, 3G, and 4G mobile communication systems have the following problems:
(1) unsuitableness for ‘always on’;
(2) long state transmission time due to contention-based random access;
(3) a necessity to continuously monitor a downlink shared control channel (SCCH) for downlink channel access;
(4) a limited number of mobile stations available in each state of the MAC layer; and
(5) inefficiency in the light of power saving.
As described above, the operational states of the MAC layers proposed up to now have many problems. Accordingly, there are demands for operational states suitable to the MAC layer of the 4G mobile communication system, which is a future mobile communication system.